The present invention relates to catalytic dehydrogenation of paraffins using a catalyst which preferably contains a crystalline zeolite, and a thermally stable polymer-porous solid membrane capable of separating hydrogen from the dehydrogenation reaction effluent. Dehydrogenation is a well-known reaction wherein paraffins are converted to olefins With C.sub.6.sup.+ hydrocarbons dehydrogenation is generally accompanied by dehydrocyclization and aromatization reactions. With C.sub.2 -C.sub.5 alkanes, dehydrogenation reactions follow different reaction pathways, depending on molecular type. Reaction equilibria reflect these differences in reaction pathways. It is important, therefore, to provide a catalyst and process conditions specifically for the dehydrogenation of a light paraffinic hydrocarbon feed.
The overall objective of this invention is to use thermally stable polymer-porous solid composite membranes to improve the conversion achievable at a given temperature in the equilibrium-limited catalytic dehydrogenation of paraffins.
The catalysts of the present invention are dehydrogenation catalysts. Preferably the catalyst comprises a zeolite, and more preferably the zeolite contains a specific quantity of alkali and/or alkaline earth components. Dehydrogenation catalysts containing alkali or alkaline earth components are known. In C. N. Satterfield, Heterogeneous Catalysis in Practice, New York: McGraw-Hill Book Company, 1980, p. 269, an Fe.sub.2 0.sub.3 --Cr.sub.2 O.sub.3 --K.sub.2 CO.sub.3 butene dehydrogenation catalyst is described, in which the potassium component helps to maintain catalyst activity by promoting the reaction between steam in the feed and coke deposited on the catalyst.
U.S. Pat. No. 4,124,649 to Rausch discloses a porous, non-acidic carrier material containing a platinum or palladium component, a rhodium component, and a tin component for use in dehydrogenation. The non-acidic carrier material contains about 0.1 to about 5 wt % of an alkali metal or alkaline earth metal. Lithium and potassium are preferred. It is taught that the function of the alkali/alkaline earth component is to neutralize any of the acidic material which may have been used in the preparation of the dehydrogenation catalyst.
U.S. Pat. No. 4,438,288 to Imai and Hung describes a dehydrogenation catalyst containing a platinum group component, a porous support material, and an excess of an alkali or alkaline earth component relative to the platinum group component. This catalyst is taught as being particularly useful for dehydrogenating paraffins having from 2 to 5 or more carbon atoms to the corresponding mono-olefins or for dehydrogenating mono-olefins having 3 to 5 or more carbon atoms to the corresponding di-olefins.
Crystalline molecular sieve zeolites have also been disclosed for dehydrogenation of paraffinic hydrocarbons. As with the art cited above, which teaches use of a non-crystalline dehydrogenation catalyst, the acidity of the zeolitic-containing dehydrogenation catalysts is an important variable. For example, U.S. Pat. No. 4,665,267 and U.S. Pat. No. 4,795,732, both to Barri teach using a catalyst having a silicalite support and containing a platinum group metal for the dehydrogenation of C.sub.2 to C.sub.10 paraffins. The catalyst of Barri is substantially free of alkali and alkaline earth metals.
U.S. Pat. No. 4,401,555 to Miller is directed to olefin production from paraffins using silicalite having a low sodium content. The silicalite used in the '555 process contains less than 0.1 wt % sodium and is composited in a matrix which is substantially free of cracking activity. Also, the composite has no hydrogenation component. According to the '555 process, the paraffinic feed may be hydrotreated to reduce sulfur levels to less than 100 ppm organic sulfur.
An intermediate pore size crystalline silicate having a high silica to alumina ratio, a relatively low alkali content, and a small crystallite size is taught as a sulfur tolerant reforming or dehydrocyclization catalyst in International Patent Application WO91/13130.
Other non-acidic catalysts have been proposed for dehydrogenation of paraffins. In U.S. Pat. No. 4,962,250, a non-acidic MCM-22 zeolite, in combination with a Group VIII metal species, is taught for dehydrogenation of C.sub.2 -C.sub.12 aliphatic hydrocarbons. In order to be non-acidic, the '250 reference teaches that the finished catalyst should contain cation equivalents of Group IA and/or IIA cations equal to or greater than the framework aluminum content.
In U.S. Pat. No. 4,929,792 to Dessau, a zeolite Beta in non-acidic form is disclosed for dehydrogenation of a C.sub.2 -C.sub.12 paraffin-containing feed. To render the Beta zeolite non-acidic, '792 teaches titrating the zeolite with Group IA or IIA in ion-exchangeable form until a Ph of greater than 7 is achieved.
Dehydrogenation processes in which hydrogen is separated from the dehydrogenation reaction zone are known. Processes available to the art include those for separating hydrogen from liquid and/or gaseous hydrocarbon streams. Such processes include distillation, adsorption, absorption, extraction and permeation through a semipermeable membrane. For example, J. N. Armor, Applied Catalysis, 49, 1 (89) describes separation processes for recovering a purified hydrogen stream from hydrogen/hydrocarbon mixtures using a semipermeable membrane. Examples of membranes which have been used include metal or metal alloy of high permeability to hydrogen (e.g., Pd, Pd/Ag), either alone as a thin foil or as a thin film on a support also permeable to hydrogen (e.g., porous ceramic, glass). Non-metallic inorganic membranes and polymer membranes are also known to the art.
Polymer-ceramic composite membranes are also known. M. E. Rezac and W. J. Koros, Journal of Applied Polymer Science, 46, pp. 1927-1938 (1992) disclose the preparation of polymer-ceramic composite membranes comprising essentially defect-free, thin (&lt;1 82 m) dense-skinned organic-inorganic composite membranes. These membranes are useful for gas separations.
The polymers used by Rezac and Koros for the production of the dense organic separating layer of their composites were (1) a fluorine-containing dianhydride-diamine polymer, 4,4'[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofuran-d ione, isopropyl idene-dianiline (6FDA-IPDA); (2) bisphenol-A polycarbonate (PC); (3) tetramethylhexafluorobisphenol-A polycarbonate (TMHFPC); (5) tetramethylhexafluoropolysulfone (TMHFPSF) and (6) 4,4'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofuran- dione, methylene-dianiline (6FDA-MDA) .
Thus, while catalytic dehydrogenation processes are known, including those which utilize a semipermeable membrane to separate hydrogen, there still exists a need for more efficient processes which provide higher conversion of paraffin to olefin. The present invention provides such a process in which conversions of paraffin to olefin are significantly higher than with conventional systems while at the same time operating several hundred degrees Fahrenheit lower than do the conventional systems which typically operate at about 450.degree. C. (842.degree. F.) to about 700.degree. C. (1292.degree. F.).